The long-term objective of the research proposed in this revised Competing Renewal application is to attain a detailed understanding of skin barrier function so that the transdermal delivery of suitable drugs may be enhanced and optimized. Because of a continued need for safe and efficacious agents to regulate fertility, applications of the basic mechanistic studies described will ultimately address the transdermal delivery of contraceptive steroids. The specific aims of the project are: [1] To determine, in vivo in man, using reflectance infrared (IR) spectroscopy, the dependence of skin (and, particularly, stratum corneum (SC)) permeation on drug structure and properties. [2] To evaluate, (again) in vivo using reflectance IR, the effects of 'penetration enhancers' on SC barrier function and drug permeability. 13] To interpret the IR data, together with other biophysical measurements, to determine the mechanism(s) of enhancer action on the SC. [4] To develop and optimize a human keratinocyte, 'air/liquid', culture system, which mimics epidermal barrier function physically, chemically, and morphologically. [5] To establish that (i) the biophysical properties, and (ii) the permeability, of the SC of the culture are comparable to those of human skin. [6) To study the concurrent transport and metabolism of drugs in the keratinocyte culture system. The principal experimental strategies involve: (a) the use of attenuated total reflectance IR spectroscopy to probe, in vivo, the biophysical basis of SC barrier function, the pathways of drug permeation across the SC, and the mechanism(s) of penetration enhancer action; and (b) the development of an 'epidermal-equivalent' keratinocyte culture system, which resembles the in vivo tissue in morphology, SC lipid composition, biochemistry, metabolism, and barrier function. Studies have been designed, therefore, to examine, determine, and correlate the physical, chemical and biological parameters which define skin (and, specifically, SC) barrier function. Ultimately, the research proposed is intended to answer the following questions: (1) What are the mechanism(s) (pathways) of transdermal penetration? (2) How is the cutaneous barrier to drug permeation constructed? What are the key chemical and biophysical constituents and requirements of a functional barrier? (3) Is it possible to reversibly subvert the SC barrier with penetration enhancers? How do these agents work? (4) Can the basic information derived be applied to solving the important problem of contraceptive steroid delivery by the transdermal route?